enable static pin as parameter for begin()

[SvOlli]

To have a simple way of protected pairing, an optional parameter can be
specified to used as a pin for authentication. Tested on a Linux system.

Signed-off-by: Sven Oliver Moll [email protected]

[T-vK]

Thanks for the pull request. I'm not sure if having the pin be the first parameter of the begin method is the best way of doing this. It could be though, I haven't really looked into it, but have you considered making the pin a constructor parameter, or maybe have a dedicated setPin method or simply a property and if so, what conclusion did you come to?
I'm a bit hesitant because once we add this, there is no way to maybe have something more important as the first parameter of the begin method in the future without breaking backwards compatibility.

[SvOlli]

The idea of using begin() was to have a similar to the begin() of the Serial class. Using the constructor is not a good idea, because the object is usually created before running main(), but you might want to read the PIN from EEPROM (simulated by flash). It is also necessary that the PIN is set before running the Bluetooth task, or the parameter is not used.
So the best thing that could be done is to create a setPIN() function that prints out an error if the Bluetooth task is already running, but that's not as elegant as the optional parameter for begin(). Since I only had an insight on my small change, but you have a much better overview of the whole library. So I'll implement whatever you think is best.
How about a separate beginPIN() method that will set the PIN and calls begin() afterwards?